It is desirable in the switched mode power supply art for the size of the power supplies to be as small as possible. However, in order to reduce the size of such power supplies, their operating frequency must be increased to a relatively high level. However, when the operating frequencies exceed 100 kilo Hertz, for example, the parasitic impedances of the various components of the power supply, such as the parallel capacitance of the electronic switches, and the leakage inductance of the transformers, become significant and can no longer be regarded as negligible second order quantities.
In the conventional prior art switched mode power supply, the parasitic impedances referred to above, must be discharged and reset during each operational cycle. This creates commutation losses, adds voltage and current stresses to the switches, and decreases the overall efficiency of the power supply. Accordingly, any attempt to decrease the size of the usual prior art switched mode power supply by increasing its operational frequency generally has not been completely successful.
Resonant switch mode power supplies have been proposed in the prior art in an attempt to minimize the parasitic impedance effects as the operating frequency is increased. However, the prior art switched mode resonant power supplies are subject to certain drawbacks. For example, the switching frequency is not stable and has a tendency to vary with changes in load and line voltage.
Also, in order to minimize the size of the resonant elements in the resonant power supplies, the switching frequency is sometimes increased to over 1 megaHertz, and this results in ferrite commutation losses.
In addition, the switching elements of the usual prior art switched mode resonant power supplies are often stressed with peak voltages and/or currents of up to three times normal values.
There is also a substantial amount of recirculating energy in the prior art switched mode resonant power supplies which is not transferred to the output. This unused recirculating energy creates losses which militates against the achievement of a small size efficient power supply.